One approach to the preparation of an aqueous-dispersible polyurethane composition, is to dissolve a polyurethane in solvent and then convert the polymer solution into a dispersion by adding the solution with agitation into water containing an emulsifier. This procedure generally suffers from several disadvantages, one of which is the use of large amounts of solvents to dissolve the polyurethane, which solvent then has to be removed. Also, the dispersions obtained by this method can be relatively unstable, and the emulsifier cannot be removed from the polymer film, these problems leading to undesirable film properties.
Another method is to synthesize the polyurethane dispersions with emulsifiers which are chemically incorporated into the polymer backbone, such as disclosed in U.S. Pat. No. 3,479,310 to Dieterich et al. Aqueous polyurethane dispersions free of emulsifiers are prepared from a polyurethane with salt-type groups incorporated into the polymer backbone. These ionic groups exert an emulsifying effect that enables an aqueous dispersion to be produced without the addition of emulsifiers or other substances which assist in the dispersion or emulsification process.
Polyurethanes containing such ionic groups can be prepared by reacting organic polyisocyanates with compounds containing at least two active hydrogen atoms in a molecule which contains at least one salt-type group or a group that is capable of salt formation. The polyurethane dispersion is preferably prepared in a suitable inert organic solvent and is dispersed in water. The properties of the polyurethane dispersions depend on the ratio of isocyanate groups to active hydrogen atoms. The term "active hydrogen atoms" refers to hydrogen atoms which, because of their position in the molecule, display activity according to Zerewitinoff test (J. Amer. Chem. Soc. 49, 3181 (1927). These include hydrogen atoms attached to 0, S and/or N as in the groups --OH, --SH, =NH, --NH.sub.2 and the like.
If the isocyanate groups are in equivalence or below the equivalent amount of the active hydrogen atoms, no further chemical reaction takes place during the conversion into an aqueous dispersion. If the isocyanate groups are in excess, however, they will react with water and give rise to lumpy or curdled dispersions. Chain extenders that are soluble in water and are highly reactive towards isocyanate groups can be used in an attempt to control the reaction, as well as to obtain high molecular weight polyurethanes.
There remains a need, however, for emulsifier-free polyurethane dispersions, particularly for use in adhesive formulations. The present invention provides novel, aqueous polyurethanes which do not require emulsifiers to form stable dispersions.